The SuperCard DSTWO has RTC, but it might not work for every game (the official website claims "Pokemon supports real-time clock" but I don't know which pokemon game(s), nor if a hacked pokemon game will work).

EDIT: never mind the DSTWO, I didn't realize it was in the GBA flashcart section. >_>

You can also try various PC emulators, as most of them (like no$gba or VBA) has RTC, which usually corresponds to your PC's time.

if you want to play on a gba/ds then you'll want to hunt down a m3 lite perfect. that will be your best option. the card sold new for $99-130. used you are going to pay around that much, because its pretty rare now. it came packaged in a ds lite shell, the gba shell was never sold separately and only some resellers made the effort to package it with the kit. make sure you ask if it has the gba shell. a few gbatemp members have sold them in the past, you might still be able to find one on this site.

i'm sure there are a few other kits that had RTC but the m3 lite perfect was one of the best slot-2 i ever used.